1. Field of the Invention
This invention relates to a process producing .alpha.-keto-carboxylic acid ester of the general formula (1), ##STR1## wherein R.sub.1 represents an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group having four or more carbon atoms and R.sub.2 represents a lower alkyl group.
The .alpha.-keto-carboxylic acid esters of the general formula (1) produced according to the process of the invention are useful as intermediates for preparing pharmaceutical drugs.
2. Description of the Related Art
Previously, .alpha.-keto-carboxylic acid esters are known as prepared, for example, according to the following processes.
(1) Process by oxidizing reaction of .alpha.-hydroxycarboxylic acid esters ##STR2## J. W. Conforth, Org. Syn. coll. Vol. 4, 467 (1963). ##STR3## S. Bergstrom, et al., Acta Chem. Scand., 6, 1157 (1952). ##STR4## T. Miyazawa, et al., J. Org. Chem., 50, 1332 (1985).
(2) Process by esterifying reaction of .alpha.-keto-carboxylic acid ##STR5## A. Weissberger, et al., Org. Syn. coll. Vol. 3, 610 (1943).
Also .alpha.-keto-carboxylic acid is known as prepared according to the following method. ##STR6## Adickes, et al., Ann., 555, 41 (1943).
(3) Process by Grignard's reaction ##STR7## L. M. Weinstock, et al., Synthetic Commun., 11 (12), 943 (1981).
These aforementioned prior art processes of preparing .alpha.-keto-carboxylic acid esters have had following problems. In the process of using readily available .alpha.-hydroxycarboxylic acid as a starting material, the use of a toxic reagents such as potassium permanganate or chromic acid, or the use of an expensive reagent such as oxoaminium salt and the like in an amount of one or more equivalents are required. In the process of esterifying .alpha.-keto-carboxylic acid, .alpha.-keto-carboxylic acid is not generally readily available because of a long reaction sequence for preparation. In the process of using Grignard's reaction, it has a problem of in low yield.
Therefore, the object of the present invention is to provide a process for producing .alpha.-keto-carboxylic acid esters from an inexpensive and readily available .alpha.-hydroxycarboxylic acid ester.